1. Field of the Invention
This invention relates to an abnormality diagnostic system for an evaporative fuel-processing system for internal combustion engines for vehicles, and more particularly to an abnormality diagnostic system which has a function of detecting abnormalities in an evaporative emission control system of the engine.
2. Prior Art
Conventionally, there has been widely used an evaporative fuel-processing system for internal combustion engines for automotive vehicles, which comprises an evaporative emission control system having a canister having an air inlet port provided therein, a first control valve arranged across an evaporative fuel-guiding passage extending from a fuel tank of the engine to the canister, and a second control valve arranged across a purging passage extending from the canister to an intake system of the engine.
An evaporative emission control system of this kind temporarily stores evaporative fuel in the canister, and then purges the evaporative fuel into the intake system of the engine.
Whether an evaporative emission control system of this kind is normally operating can be checked, for example, by bringing the evaporative emission control system into a predetermined negatively pressurized state, measuring a change in the pressure within the fuel tank (tank internal pressure) occurring with the lapse of time after the evaporating emission control system has been brought into the predetermined negatively pressurized state, by a tank internal pressure sensor which detects the tank internal pressure, and determining whether the system is normally operating, from the measured tank internal pressure, as proposed by Japanese Patent Application No. 3(1991)-262857 and corresponding U.S. Pat. No. 5,299,545, assigned to the assignee of the present application, for example.
According to the method of the earlier application, an amount of change in pressure prevailing within the evaporative emission control system is detected by the tank internal pressure sensor, to determine an abnormality in the system in such a manner that if the detected pressure change amount is below a predetermined value, it is presumed that an amount of evaporative fuel leaking from the system to the outside is small and hence it is determined that the system is normally functioning, whereas if the detected pressure change amount exceeds the predetermined value, it is presumed that evaporative fuel is leaking in a large amount from the system to the outside, and hence it is determined that the system is malfunctioning.
The determination of abnormality of the evaporative emission control system according to the method of the earlier application is carried out when predetermined abnormality determination-permission conditions are satisfied during running of the vehicle, i.e. when the engine enters a predetermined operating condition during running of the vehicle.
However, the upper surface of fuel within the fuel tank largely moves or stirs when the vehicle is in a particular running condition such as acceleration, deceleration and turning. Consequently, the pressure within the fuel tank largely changes when the vehicle is in such a particular running condition. When the pressure within the fuel tank thus largely changes due to running of the vehicle in such a particular running condition, it can be erroneously determined that the system is abnormal even when it is normally functioning.
Further, according to the method of the earlier application, to forcibly bring the interior of the evaporative emission control system into the predetermined negatively pressurized state, the second control valve is opened to communicate the interior of the system with the intake system of the engine via the purging passage. Then, a large amount of fuel vapor (evaporative fuel) is supplied into the intake system due to a gas drawing force created by the engine. This causes large fluctuations in the air-fuel ratio of a mixture supplied to the engine, and more frequent emission of unburnt gases through an exhaust system of the engine.
Moreover, the abnormality determination can be frequently carried out whenever the abnormality determination-permission conditions become satisfied, resulting in spoilage of drivability and degraded exhaust emission characteristics of the engine.